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Despite Tantalizing Hints, Voyager 1 Has Not Crossed into the Interstellar Medium

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Voyager 1 and Voyager 2 leaving the solar system

Diagram of the solar (white arrows) and interstellar (black arrows) magnetic fields in relation to the Voyager 1 (top) and Voyager 2 (bottom) spacecraft on the edge of the heliosphere. Credit: NASA/JPL-Caltech

Voyager 1 is going, going … not quite gone.

The well-traveled NASA spacecraft, launched in 1977, is headed out of the heliosphere, the fluctuating bubble in space inflated by plasma streaming outward from the sun. For years Voyager 1 has been closing in on the heliopause—the outer edge of the heliosphere—where the solar wind meets the interstellar medium. But despite intriguing hints to the contrary, the probe remains within the heliosphere, mission scientists have announced. Rather than making an unprecedented crossing into the interstellar medium, it appears that Voyager 1 has discovered yet another wrinkle in the structure of our local space environment, a sort of magnetic highway linking the heliosphere to what lies beyond.

"Voyager has discovered a new region of the heliosphere that we had not realized was there," Voyager project scientist Ed Stone of the California Institute of Technology said in a December 3 teleconference. "We're still inside, apparently. But the magnetic field now is connected to the outside. So it's like a highway letting particles in and out."

Voyager 1 crossed into the new region in August, suddenly registering a huge drop in the number of low-speed solar particles in its environment and a corresponding jump in the number of higher-energy cosmic-ray particles arriving from outside the solar system. "It is as if someone opened the floodgates and the water all moved down the river," said Tom Krimigis of the Johns Hopkins University Applied Physics Laboratory. "Also some boaters traveling upstream at close to the speed of light have been able to get in at last."

Charged particle data from Voyager 1

Data from Voyager 1 show an abrupt drop in solar ions (top) at the same time that the spacecraft detected an increased number of cosmic rays (bottom) from interstellar space. Credit: NASA/JPL-Caltech/GSFC

Despite the sudden influx of cosmic-ray particles, the team concluded that Voyager 1 is still inside the heliosphere because the probe's magnetometer has not yet registered a change in magnetic field direction, as would be expected when crossing the boundary from the sun's plasma to the interstellar medium. "If we had only looked at particle data alone, we would have said, 'Well, we are out. Goodbye to the solar system,'" Krimigis said. "But nature is very imaginative, and Lucy pulled up the football again."

Voyager 1 is now 18.3 billion kilometers from the sun, farther out than any man-made object has ever traveled. It is so distant that it takes radio signals 34 hours to make the round-trip (to send a command, for instance, and then receive confirmation that the command was accepted) from Earth to Voyager and back again. Still, no one knows how much farther Voyager 1 may have to travel before it breaches the heliopause and enters the interstellar medium. "It may take several more months, it may take several more years," Stone said.

The views expressed are those of the author and are not necessarily those of Scientific American.

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